1. Field of the Invention
This invention is directed to the field of medical transplants of cells and tissues, the manufacture of such transplants, and their use. In particular, this invention is directed to a process for forming thin, uniform, continuous coatings on tissue transplants such as pancreatic islet cells with a high degree of reproducibility.
2. Background and Related Disclosures
Traditional medical treatments for functional deficiencies of secretory and other biological organs have focused on replacing identified normal products of the deficient organ with natural or synthetic pharmaceutical compositions. For example, for treating insulin-dependent diabetes mellitus, also known as type I or juvenile onset diabetes, the normal secretion of insulin by the islets of Langerhans in the pancreas must be replaced since functional islets are no longer present in the pancreas. This pancreatic function is emulated by administering insulin, titrating the injections in response to blood glucose level measurements. At best, the normal production of the islets are poorly approximated.
Organ replacement has also been used. This has generally required continuous use of immunosuppressive agents to prevent immunological rejection of the organ, depriving the patient of the full protective function of the immune system against diseases. It has provided permanent relief only for a limited group of organs.
Attempts to transplant organ tissues into genetically dissimilar hosts without immunosuppression have been generally defeated by the immune system of the host. Prior to this invention, application of effective protective barrier coatings to isolate the transplant tissues from the host immune system has not proven to be medically practical for a number of reasons. The coating materials were incompatible with the host system or unsuitable for other reasons. Encapsulation or coating processes previously developed did not yield reproducible coatings having the desired permeability and thickness required for the transplant tissue to have a long and effective functioning life in the host.
To protect transplants from destruction by the immune response of the host animal, various attempts have been made to create a protective barrier between the transplant tissue or cells and the immunological components of the host's system. T. M. S. Chang, Science, 146:524-525 (1964) described the microencapsulation of erythrocyte hemolysate and urease in semi-permeable polyamide membranes. These microcapsules did not survive for long when injected into the blood stream. K. Mosbach et al, Acta Chem. Scand., 20:2807-2812 (1966) and T. M. S. Chang et al, Can. J. Physiol. and Pharmacol., 44:115-128 (1966) described the preparation of semi-permeable microencapsulated microbial cells and viable red blood cells, the latter article mentioning the possibility of using injections of encapsulated cella for organ replacement therapy.
Encapsulation methods applied to make these materials have comprised a procedure for forming droplets of the encapsulating medium and the biological material and a procedure for solidifying the encapsulating medium. Agarose encapsulated materials have been formed by chilling an emulsion of agarose droplets containing biological materials as shown by Nilsson et al, Nature, 302:629-630 (1983) and Nilsson et al, Eur. J. Appl. Microbiol. Biotechnol., 17:319-326 (1983). Injection of droplets of polymer containing biological materials into a body of coolant such as a concurrently flowing liquid stream has been reported by Gin et al, J. Microencapsulation, 4:329-242 (1987).
Alginates form a gel when reacted with calcium ions. Alginate droplets have been formed by emulsifying a solution of sodium alginate containing cellular material to form droplets of sodium alginate and cells, and gelling the droplets with calcium chloride in U.S. Pat. No. 4,352,883. Alginate droplets have also been formed with a syringe and pump to force droplets from a needle, using a laminar flow air knife to separate droplets from the tip, the droplets being gelled by collecting them in a calcium chloride solution in U.S. Pat. No. 4,407,957. Alginate droplets have also been formed by the simple procedure of expelling them from a hypodermic needle and allowing the droplets to fall into a calcium chloride solution, as described by Nigam et al, Biotechnology Techniques, 2:271-276 (1988). Droplets have also been injected into a concurrently flowing stream containing calcium chloride in U.S. Pat. No. 3,962,383. Spraying alginate solutions through a spray nozzle to form a mist of droplets which were collected in a calcium chloride solution was reported by Plunkett et al, Laboratory Investigation, 62:510-517 (1990). These methods have not proven effective for mass production of coatings required for successful transplantation.
Hommel et al in U.S. Pat. No. 4,789,550 disclose the formation of alginate droplets using a combination of a needle and a square wave electrical electrostatic voltage to form uniform droplets. The alginate solution was forced from the tip of a needle to form a droplet, and the droplet was pulled from the needle by a changing electrostatic field between the needle tip and a calcium chloride solution placed below the needle tip. The droplet received a charge of one polarity from the needle, opposite to the charge in the calcium chloride solution. When the voltage difference between the droplet and the oppositely charged calcium chloride solution reached a value at which the attraction by the solution on the droplet exceeded the force of interfacial tension holding the droplet on the needle tip, the droplet was pulled free to fall into the calcium chloride solution. The electrostatic field was fluctuated using a square wave form to create a succession of voltages crossing the threshold voltage at which droplets were pulled free from the needle, thus producing a continuous series of droplets, one per square wave cycle. The process was not found to provide the small droplets and thin coatings required for effective transplantation.